Preheat oven to 350 degrees F, and bake mix in two 8-inch pans according to package instructions. Let cool.

Make filling: Combine softened butter, confectioners' sugar, fluff, and vanilla in the bowl of a stand mixer. Beat until smooth and fluffy. Set aside.

Make ganache: Pour chocolate chips into a medium bowl. In a small saucepan over medium heat, bring heavy cream to a slow simmer. Remove from heat and pour over chocolate chips. Let sit 5 minutes, then stir until smooth. Let cool slightly.

Assemble the cake: Place one layer of cake on a platter. Spread marshmallow filling over cake, and top with other layer. Pour ganache over the cake and spread until entire cake is covered.

Apply the Swiss Rolls in even rows around the cake until no ganache is showing. Allow to set for 30 minutes before slicing.

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